This invention relates to an amplifier which is particularly useful for amplifying pulse width modulated signals and, in particular, to such an amplifier having field effect transistors which exhibit triode-type dynamic characteristics and which is of relatively simple and inexpensive construction.
In amplifiers which are to be used with pulse width modulated signals, it is desirable to minimize the switching distortion attending the switching of the amplifying elements due to the pulse signals. A preferred amplifier configuration is a push-pull amplifier. In a simple embodiment thereof, such a push-pull amplifier is provided with a pair of bi-polar transistors connected in the recognized push-pull relation. When used with a pulse signal, such as a pulse width modulated (PWM) signal, the bi-polar transistors not only must amplify the signal but also must be switched on and off in response to the PWM signal. Unfortunately, such switching of a typical bi-polar transistor is accompanied by deleterious switching distortion which is attributed to the storage carriers which are included in the bi-polar transistor.
Since the field effect transistor (FET) does not rely on such storage carriers for its operation, the FET is known to exhibit desirable switching characteristics. Hence, it may be thought that the use of an FET in, for example, a push-pull amplifier for PWM signals might be an advantageous circuit configuration. Unfortunately, most FET's which have been known heretofore exhibit a relatively small drain current capacity. This limits the amount of amplification which can be obtained therefrom and thus constrains the use of such an FET in an amplifying device.
Recently, an FET of the type having triode-type dynamic characteristics has been developed. This type of FET offers many advantageous features, such as a very small output resistance, the avoidance of drain current saturation with an increase in drain voltage and superior voltage-current characteristic linearity. Moreover, this type of FET has a larger drain current capacity than that of the prior art FET. Because of these characteristics, as well as excellent switching characteristics, this recently developed FET finds ready application in Class-B push-pull amplifiers for audio signals.
The present invention proceeds upon the use of such an FET having triode-type dynamic characteristics in an amplifier circuit for use with PWM signals. It is believed that improved results can be attained over prior art amplifiers when constructed of such FET devices. For example, in the prior art amplifier, bi-polar transistors are connected in push-pull relation to supply the amplified PWM signal to a load through a low-pass filter, such as a choke coil. At a pulse transition, when the amplified PWM signal changes its amplitude (or polarity), a reverse current is produced by the choke coil, which is a discharge current therefor. However, because the bi-polar transistors generally are not capable of providing a path for this reverse current, it is necessary to use discharge diodes which are connected across the collector-emitter circuit of each transistor. While these diodes thus serve to discharge the choke coil, they must be capable of accommodating current switching frequencies of a relatively high frequency range. As is known, such high frequency diodes are costly.